I agree with Gene. The legal route is in the road.
If courses are measured so that shortcutting is impossible, it's not curbs but building walls that become the course boundaries.
Shortcutting is a problem for race administration, not for the measurer. In most cases it's not the frontrunners who do it but a few folks back in the pack. Ultimately it makes little difference to the finish time.
It can be made clear to the runners that curbhopping is cheating, and that if you are reported you'll be disqualified.